The agricultural industry is under pressure to reduce pesticide, particularly herbicide, usage. This is evidenced by symposia on the subject, such as that held in 1993 by the Weed Science Society of America and documented in Weed Technology 1994, Vol. 8, pp. 331-86. Reduced use rates are desirable both environmentally and economically, in that the treatment cost per unit area decreases. In the case of exogenous chemicals applied to foliage of a plant, herein referred to as "foliar-applied" exogenous chemicals, enhanced delivery efficiency can also improve the ability or tendency of an exogenous chemical such as a pesticide to retain its biological effectiveness when the treated plant is exposed to natural or artificial rain or overhead irrigation within a short period (such as a few minutes to a few hours) after application. This property is generally referred to as "rainfastness." In many cases, enhanced delivery efficiency leads to earlier manifestation of outward signs or symptoms that the applied exogenous chemical is exerting its desired effect in or on a treated plant, on parasites or pathogens of the plant, or on organisms, particularly invertebrate animals such as insects, feeding on non-woody or woody parts of the plant.
Exogenous chemicals, especially foliar-applied exogenous chemicals including foliar-applied herbicides, are commonly formulated with surfactants or wetting agents, so that when water is added, the resulting sprayable composition is more easily and effectively retained on the foliage (e.g., the leaves or other photosynthesizing organs) of plants. Surfactants can also bring other benefits, including improved contact of spray droplets with a waxy leaf surface and, in some cases, improved penetration of the accompanying exogenous chemical into the interior of leaves. Through these and perhaps other effects, surfactants have long been known to increase the biological effectiveness of herbicide compositions, or other compositions of exogenous chemicals, when added to or included in such compositions. Thus, for example, the herbicide glyphosateis typically formulated with surfactants such as polyoxyalkylene or polyglycoside surfactants. More particularly, certain commercial formulations of glyphosate herbicide marketed under the trademark ROUNDUP.RTM. have been formulated with a polyoxyalkylene alkylamine, in particular polyoxyethylene (15) tallowamine.
European Patent No. 0 394 21 1 discloses solid granular formulations of glyphosate containing organosilicone wetting agents or fluoro-organic wetting agents. Some commercial formulations of glyphosate herbicide have been formulated with such surfactants, including a particular group of polyoxyalkylene polysiloxane surfactants exemplified by the commercial organosilicone surfactant Silwet L-77, which has been reported to affect the foliar absorption of glyphosate by plants. A number of studies relating to the use of Silwet L-77 with glyphosate and other herbicides have been published. It should be noted that surfactants have been combined with glyphosate or other exogenous chemicals either in a concentrate liquid or dry composition (herein referred to as a "simple coformulation") containing an intimate admixture (i.e. not partitioned in separate phases of the concentrate composition) of both exogenous chemical and surfactant, or in a diluted mixture that is prepared from separate exogenous chemical (e.g. glyphosate) and surfactant compositions immediately prior to use in the field (herein referred to as a "tank mix"). Simple coformulations and tank mixes, and methods for applying them, are herein distinguished from the "sequential application" methods that are the subject of this invention.
A foliar uptake study of glyphosate herbicide, wherein an organosilicone surfactant (Silwet L-77) was applied together with glyphosate to simulate a tank mix, is reported by Field & Bishop, Pestic. Sci., 1988, Vol. 24, pp. 55-62. When these tank mix compositions were applied to the adaxial leaf surfaces of perennial ryegrass plants, complete surface wetting was observed at Silwet L-77 concentrations of 0.1-0.5% by volume. Through timed experiments wherein radio-labeled glyphosate was applied to the leaves followed by washing of the leaves, it was concluded that use of Silwet L-77 provides a reduced critical rain-free period after application because of an enhanced rate of glyphosate uptake. Rapid uptake was observed into stomata of the plants treated with the tank mix. Visual confirmation of stomatal uptake was confirmed by dye studies. However, these workers found Silwet L-77 antagonistic to glyphosate uptake over a 48 hour period. Herbicidal effects were reported in terms of tiller regrowth (expressed as percentage of tiller number at time of glyphosate application). Stevens et al, Pestic. Sci., 1991, Vol. 33, pp. 371-82, note an enhancement of herbicide uptake over a 0-6 hour period for tank mixes of glyphosate and Silwet L-77.
Another study of the effects of Silwet L-77 upon the foliar uptake of glyphosate herbicide is reported in an article by Gaskin & Stevens, Pestic. Sci., 1993, Vol. 38, pp. 185-92. In this study, radio-labeled glyphosate (specifically the isopropylammonium salt of glyphosate) was utilized to determine the uptake of herbicide in wheat plants. The authors measured the foliar uptake when Silwet L-77 was applied before (pretreatment), during (i.e., in a tank mix), and after (post-treatment) application of the glyphosate herbicide to the plants. Pretreatment of the plants with Silwet L-77 reduced the uptake of glyphosate by the foliage over the course of the study and generally failed to increase even the initial rate of uptake of glyphosate into the plant. Both simultaneous (i.e., tank mix) and post-treatment of the plants with Silwet L-77 at 4 and 8 hours after herbicide application were found to increase the initial rate of uptake of glyphosate; but these workers concluded that "the initial enhancements provided by both simultaneous and sequential application of Silwet L-77 slowed down rapidly thereafter in all treatments." The article reports no measurements of herbicidal effectiveness for any species. The article states that Silwet L-77 may be beneficial as a spray (i.e., a tank mix) adjuvant if rain falls after its application but not in the absence of rain. Further study of the antagonism of glyphosate uptake by Silwet L-77 is reported by Gaskin & Stevens, Pestic. Sci., 1993, Vol. 38, pp. 193-200.
An extensive review of 160 citations relating to the use of organosilicone surfactants as adjuvants for agrochemicals was provided by Stevens, Pestic. Sci., 1993, Vol. 38, pp. 103-22. Stevens reviews work reporting the use of organosilicone surfactants in formulations of herbicides, foliar nutrients, growth regulators, insecticides, and fungicides. Although Stevens discusses extensively work relating to coformulations or tank mixes of organosilicones with, e.g., herbicides, there is no discussion of work relating to sequential application of these materials.
The effects of Silwet L-77 on the foliar uptake of other herbicides has been investigated. Buick et al., Pestic. Sci., 1993, Vol. 38, pp. 227-35, report increases in uptake of triclopyr triethylamine in field bean over time periods of one hour and six hours by inclusion of Silwet L-77 in a simulated tank mix. These workers posit infiltration of foliar stomata to explain this effect. Other workers have questioned the significance of stomatal infiltration to the operation of organosilicone surfactants. Roggenbuck et al., Weed Tech., 1994, Vol. 8, pp. 582-85, conclude there is no relationship between the number of stomata covered and the degree to which herbicide uptake is influenced by addition of Sylgard 309, an organosilicone surfactant.
Antagonistic effects with respect to the herbicidal effectiveness or uptake of glyphosate have been reported in the following species for tank mixes containing Silwet L-77:
colonial bentgrass (Agrostis tenuis) PA1 downy brome (Bromus tectorum) PA1 orchardgrass (Dactylis glomerata) PA1 crabgrass (Digitaria sp.) PA1 barnyardgrass (Echinochloa crus-galli) PA1 goosegrass (Eleusine indica) PA1 quackgrass (Elymus repens) PA1 wild poinsettia (Euphorbia heterophylla) PA1 common velvetgrass (Holcus lanatus) PA1 dallisgrass (Paspalum dilatatum) PA1 prostrate knotweed (Polygonum aviculare) PA1 green foxtail (Setaria viridis) PA1 johnsongrass (Sorghum halepense) PA1 wheat (Triticum aestivum) PA1 cocklebur (Xanthium pennsylvanicum)
See Gaskin & Stevens, Pestic. Sci., 1993, Vol. 38, pp. 185-92; Baylis & Hart, Brighton Crop Protection Conference, 1993, pp. 1331-36; Field & Tisdall, Ninth Australian Weed Conference, 1990, pp. 332-35; Australian Patent Publication No. 38389/89.
Blumrhorst & Kapusta, Weed Technolog, 1987, Vol.1, pp. 149-53, have investigated sequential and tank mix applications of plant growth regulators (specifically, mefluidide) with herbicides. A study of the sequential application of herbicide materials is reported by Qureshi & VandenBorn, Canadian Journal of Plant Science, 1979, Vol. 59, pp. 93-98.
Because surfactants can enhance herbicidal effects when coformulated with or added in a tank mix to herbicidal compositions, numerous workers have studied the effects of various surfactants. One extensive study was conducted by Wyrill & Burnside, Weed Science, 1977, Vol. 25, pp. 285-87. These investigators concluded that "the effectiveness of surfactant combinations was quite variable and difficult to predict. Therefore, the indiscriminate addition of surfactants into glyphosate spray mixtures which already contain a surfactant should be avoided." However, this study did not include any organosilicone or fluoro-organic surfactant treatment. Another study of surfactant effects on glyphosate is set forth in Gaskin & Kirkwood, Adjuvants and Agrochemicals, 1989, Vol. 1, Chapter 13, pp. 129-39. In this study, surfactants (including Silwet L-77) are compared and rated for selected herbicides, based upon plant uptake and translocation measurements. Silwet L-77 was shown to be superior to two non-organosilicone surfactants for enhancing glyphosate uptake and translocation in bracken when added to the glyphosate spray solution as a tank mix.
So many studies are reported in this area that OSi Specialties (a unit of Witco Corporation) has published a Bibliography of Silwet Organosilicone Surfactants As Agricultural Adjuvants (1996), which is indexed for computer searching. This bibliography lists hundreds of published studies of commercial organosilicone surfactants in agricultural applications. This bibliography is available to the public through the publisher's office in Tarrytown, N.Y.
Bishop & Field, Aspects of Applied Biology, 1983, Vol. 4, pp. 363-70, report that Silwet L-77 in tank mix enhanced the performance of glyphosate in field trials on perennial ryegrass. "Spectacular" leaf wetting was observed for tank mixes including 0.5% by volume Silwet L-77, indicating pronounced spreading of the herbicide over the foliar portions of the plant. Stevens et al., Pestic. Sci., 1991, Vol. 33, pp. 371-82, report that in vicia bean leaves, stomatal infiltration of Silwet L-77 is antagonized by the surfactant coformulant in ROUNDUP.RTM. herbicide. Baylis & Hart, Brighton Crop Protection Conference, 1993, pp. 1331-36, have concluded that the effect of Silwet L-77 in tank mix on the herbicidal efficacy of glyphosate-trimesium (the trimethylsulfonium salt of glyphosate) varies with plant species, and could not be explained simply by stomatal infiltration.
Many have investigated the possible mechanisms of herbicide antagonism by Silwet L-77 and, therefore, the means to avoid it. As used herein, "antagonism" refers to a decrease in biological (such as herbicidal) effectiveness of an exogenous chemical (such as a herbicide) when a material (such as Silwet L-77) is used in combination with the exogenous chemical; although it has been used in some of the literature cited herein to refer to a decrease in uptake or translocation. Reduction of antagonism is believed to be one of the means by which the sequential application method of this invention improves the result obtained through tank mixes or coformulations of surfactants with exogenous chemicals.
Australian Patent Application No. 38389/89 reports the use of tank mixed formulations of glyphosate and Silwet L-77, in combination with a humectant such as glycerin. An uptake investigation of similar formulations is reported by Field & Tisdall, Ninth Australian Weed Conference, 1990, pp. 332-35. Glycerin was claimed to promote the uptake of glyphosate from formulations containing Silwet L-77. In this study, paspalum leaves were treated with formulations containing Silwet L-77, with and without glycerin. Pretreatment of the paspalum leaf surfaces with Silwet L-77 two hours prior to application of glyphosate stimulated uptake. Silwet L-77 tank mixed with glyphosate did not. These investigators stated that "glycerin does not appear to have a pronounced humectant effect and it is concluded that antagonism and its alleviation by glycerin involves specific leaf surface - solution interactions that are clearly species specific." They concluded that no stomatal infiltration occurred even at Silwet L-77 concentrations as high as 0.5% by volume.
From the numerous publications on the subject of formulating exogenous chemicals such as glyphosate herbicide with various surfactants, particularly organosilicone surfactants and others that can induce stomatal infiltration, it must be concluded that the effects observed vary with the plant species, exogenous chemical, and surfactant. Tank mixed formulations containing Silwet L-77 (or other surfactants) can yield improved results on some species, but frequently antagonize the biological effectiveness for others. In the case of herbicides, this provides a disincentive to use surfactants like Silwet L-77, because multiple weed species are typically treated in the same field and the surfactant is likely to prove antagonistic for at least some of the weed species present. Similar disincentives hold for other classes of exogenous chemicals.
The problem addressed by the present invention can be stated in its broadest sense as follows. Significant benefit in the efficiency of delivery to the interior of a plant, and therefore in the ultimate biological effectiveness in the plant, of an exogenous chemical can often be obtained, as shown in the art, by adding a stomatal infiltrant such as an organosilicone surfactant in tank mix or simple coformulation to the exogenous chemical. However, this benefit is offset by a risk that the stomatal infiltrant will antagonize, rather than enhance, the biological effectiveness of the exogenous chemical. The occurrence of such antagonism is largely unpredictable. A method that consistently reduced such antagonism whenever it occurred, or that substantially removed the risk of antagonism, while still offering the benefit of enhanced delivery sought from the stomatal infiltrant, would be a great advance in the art.